1. Field of the Invention
The present invention relates generally to photoelectric devices that are used to detect the presence of objects which obstruct a beam of light and, more particularly, to a photoelectric system that is capable of changing a threshold magnitude in response to changes in light intensity of a light beam when no objects are obstructing the beam.
2. Description of the Prior Art
Many photoelectric detection systems are known to those skilled in the art. In a typical photoelectric detection system, a light source is provided which directs a beam of light that is received by a light sensitive component. For example, a light emitting diode can be used as a light source and a photodiode or phototransistor can be used as the light sensitive component. The light sensitive component can be configured within a separate housing and disposed so that a beam of light is directed from a first housing with the light emitting diode within it to a second housing with the photosensitive component in it or the light emitting diode and the photosensitive component can be disposed within a common housing. In this later type of photoelectric device, a reflector can be used to receive the light beam from the light source and reflect it back toward the light sensitive component. In typical sensors of this type, a received light signal is compared with a preselected fixed threshold magnitude. If the receiving signal is less than the threshold, the presence of an object is detected. On the otherhand, if the signal is greater than the threshold, the sensor determines that no object is present in the path of the light beam. Therefore, a threshold must be selected in such a way that its magnitude is less than the signal level when no object is obstructing the beam path but greater than the signal level when an object is obstructing the beam path. If the system is intended to detect opaque objects, a threshold can be preselected and set to a value that is a very small percentage of the unobstructed light beam intensity. If the photoelectric system is intended to detect the presence of transparent objects, such as clear glass or plastic bottles, this type of preselected threshold can be severely disadvantageous. Since transparent objects decrease the intensity of light received by the light sensitive component by a relatively small amount, the preselected threshold must be set to a value that is almost equal to the unobstructed light beam intensity. This creates a situation in which changes in the transmitted light intensity can product false signals even when no object obstructs the light beam.
U.S. Pat. No. 5,008,531, which issued to Ono et al on Apr. 16, 1991, discloses a pulsed light identifying system which includes a photoelectric conversion unit for receiving a pulsed light from a light emitting part of a light receiving part to generate a light reception signal that varies in magnitude according to the quantity of the received light. A peak holding circuit is provided with a time constant that is relatively long in comparison with the pulse interval of the pulsed light. The peak holding circuit stores a peak value of the light reception signal for its time constant. The peak value is divided by a constant for application to a comparator. The comparator compares it with the light reception signal as a threshold. A peak value of the light reception signal is held and the peak value thus held is specified as a threshold to the light reception signal. Therefore, in the event of a decrease in the quantity of emitted light due to a secular change or of decrease in the quantity of received light due to mist and oil film, the threshold decreases accordingly.
U.S. Pat. No. 5,008,532, which issued to Ono et al on Apr. 16, 1991, discloses a light reception signal circuit for a photoelectric switch. The circuit includes a photoelectric conversion circuit for receiving light from a light emitting part controlled for periodic emission. The photoelectric conversion circuit generates an electrical signal corresponding to the quantity of the received light. A programmable attenuator receives the output electrical signal and attenuates it in even step in response to a digital control signal. A first comparator compares the output voltage with a predetermined upper bound threshold. The first comparator generates a clock signal when the output voltage exceeds the upper bound threshold. A peak holding circuit for receiving an output voltage of the programmable attenuator holds the peak value of the output voltage on a time constant that is much longer than an emission period of the light emitting part. A second comparator compares the output voltage of the peak holding circuit with a predetermined lower bound threshold and generates an outgoing signal when the output voltage decreases below the lower bound threshold. A counter counts the clock pulses thereby generating a digital control signal for increasing the attenuation of the programmable attenuator. A third comparator compares the output voltage of the programmable attenuator with a predetermined decision level to produce the detection output of the photoelectric switch.